[PATCH v2 1/7] Compiler Attributes: Add __alloc_size() for better bounds checking

Kees Cook
2021-08-18T21:40:39Z

GCC and Clang can use the "alloc_size" attribute to better inform the
results of __builtin_object_size() (for compile-time constant values).
Clang can additionally use alloc_size to inform the results of
__builtin_dynamic_object_size() (for run-time values).

Because GCC sees the frequent use of struct_size() as an allocator size
argument, and notices it can return SIZE_MAX (the overflow indication),
it complains about these call sites may overflow (since SIZE_MAX is
greater than the default -Walloc-size-larger-than=PTRDIFF_MAX). This
isn't helpful since we already know a SIZE_MAX will be caught at run-time
(this was an intentional design). Instead, just disable this check as
it is both a false positive and redundant. (Clang does not have this
warning option.)

Cc: Miguel Ojeda <ojeda@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: clang-built-linux@googlegroups.com
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Makefile | 6 +++++-
include/linux/compiler_attributes.h | 6 ++++++
2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 72f9e2b0202c..34cffcdfd5dc 100644
--- a/Makefile
+++ b/Makefile
@@ -1078,9 +1078,13 @@ KBUILD_CFLAGS += $(call cc-disable-warning, stringop-overflow)
# Another good warning that we'll want to enable eventually
KBUILD_CFLAGS += $(call cc-disable-warning, restrict)

-# Enabled with W=2, disabled by default as noisy
ifdef CONFIG_CC_IS_GCC
+# Enabled with W=2, disabled by default as noisy
KBUILD_CFLAGS += -Wno-maybe-uninitialized
+
+# The allocators already balk at large sizes, so silence the compiler
+# warnings for bounds checks involving those possible values.
+KBUILD_CFLAGS += -Wno-alloc-size-larger-than
endif

# disable invalid "can't wrap" optimizations for signed / pointers
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 67c5667f8042..203b0ac62d15 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -54,6 +54,12 @@
#define __aligned(x) __attribute__((__aligned__(x)))
#define __aligned_largest __attribute__((__aligned__))

+/*
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-alloc_005fsize-function-attribute
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#alloc-size
+ */
+#define __alloc_size(x, ...) __attribute__((__alloc_size__(x, ## __VA_ARGS__)))
+
/*
* Note: users of __always_inline currently do not write "inline" themselves,
* which seems to be required by gcc to apply the attribute according
--
2.30.2